Nowadays, integrated circuits (ICs) may comprise a plethora of sensors, such as gas sensors, relative humidity (RH) sensors, specific analyte detection sensors, and so on. Such sensors may be included in the IC design for a number of reasons.
For instance, a gas sensor may be included in an IC to detect a change in the ambient conditions of a product tagged with the chip such that product quality control can be achieved by monitoring the sensor readings of the chip. This can for instance be used to accurately predict the remaining shelf life of the product, e.g. perishable food stuff. The gas sensor may for instance be adapted to determine changes in the CO2 content of the ambient atmosphere. Alternatively, the gas sensor may be used to detect changes in the gas composition of larger environment such as buildings or may be used in medical application domains, e.g. in breathing apparatuses.
With the on-going diversification of electronic devices or electronic information gathering such as by RF tags on packaged articles, it is often desirable to include different types of sensors in a single IC. For instance, the detection of other environmental parameters, for instance temperature and humidity such as for HVAC (heating, ventilation and air conditioning) control in buildings and cars, are particularly desirable in certain application domains. In addition, sensing of analytes of interest, e.g. CO2, may be desirable in such application domains. However, it is difficult to manufacture CO2 sensors having the desired sensitivity in a cost-effective manner. In particular, impedometric CO2 sensors, i.e. sensors based on measuring the change in the impedance of a material based on its exposure to CO2, suffer from relatively poor sensitivity or at least poor responsiveness.
A potentially interesting approach to CO2 sensing is to use materials that exhibit a change in a physical property upon reacting with CO2. Such materials are known per se. For instance, in NRL Report 6047 “Filament-winding plastics Part 1—Molecular Structure and Tensile Properties” of Mar. 16, 1964 and retrieved from the Internet: https://torpedo.nrl.navy.mil/tu/ps/pdf/pdf_loader?dsn=7590632 on Tuesday 7 Aug. 2012 it is disclosed that m-xylylene diamine and an epoxy resin containing it have a tendency to cloud as the amine absorbs carbon dioxide from the atmosphere.
Moreover, A. Dibenedetto et al. in ChemSusChem, Special Issue: 2nd EuCheMS Chemistry Congress, Volume 1, Issue 8-9, pages 742-745, Sep. 1, 2008 disclose the reversible uptake of CO2 from simulated flue gases by mono- and disilyl amines, either in their free form, as organic (wet) solutions, or as xerogels.
Liu et al. in Science, Vol. 313 (2006) pages 958-960 disclose a surfactant including amidine functional group that can be switched upon exposure to CO2 to a clouded amidinium bicarbonate salt.
Darwish et al. in Chem. Eur. J. 17 (2011), pages 11399-11404 disclose a spiropyran amidine that exhibits a colour change upon formation of the amidinium bicarbonate salt following reaction with CO2.
David J. Heldebrandt et al. in Energy Procedia 1 (2009), pages 1187-1195 disclose a new class of CO2 absorbing materials, referred to as CO2-binding organic liquids (CO2-BOLS) that are neat (solvent-free) liquid mixtures of organic alcohols and organic amidine or guanidine bases, which undergo the following reversible reaction in the presence of CO2:

The CO2 may be released by purging the CO2-BOLS e.g. with N2. In the above reaction scheme, DBU (diazabicyclo[5,4,0]undec-7-ene) is shown as the amidinium precursor, although Heldebrandt et al. disclose that a large variety of amidines and guanidines exhibit similar sensitivity to CO2. Similarly, several alkyl alcohols, e.g. hexanol, may be used to form the alkylcarbonate anion. The CO2 uptake was determined using conductivity measurements of the CO2-BOLS dissolved in acetonitrile, as the level of CO2 uptake is (linearly) correlated to the conductivity of the solution. The authors recommend that the absorption capacity of a CO2-BOLS can be increased by choosing a base and alcohol of low molecular weight, e.g. 1,1,3,3-tetramethylguanidine (TMG) and methanol.
The above disclosures have in common that the reversible reaction of the organic compound with CO2 causes a significant change in the charge distribution of the organic compound due to the formation of an ammonium and carboxylate ion pair, which change could potentially be used for sensing purposes, e.g. using transduction principles that could potentially quantify the number of formed ions or ion pairs, as for instance demonstrated in the paper by Heldebrandt et al.
However, exploratory experiments have highlighted that transduction measurements on such ionic liquids are typically restricted to frequencies in the Hz or kHz domain and suffer from a cross-sensitivity to the amount of water present in the sample, which makes this solution less attractive given the cross-sensitivity and the limitations on the speed of the CO2 sensor.